1. Field of the Invention
The present invention relates to electro-thermal initiated explosive devices, and in particular, to a method of forming a bridge wire resistance element, integral with a composite primary and secondary set of charges, that can be directly applied to a flexible substrate.
2. Related Art
Electro-explosive devices (EED's) are well known in the art and include such devices as primers, detonators, and squibs. These devices generally include a pair of lead wires which are connected through a bridge wire or other bridge element that is supported on a generally flat surface and contacted with a primary explosive, i.e. a sensitive, deflagration charge. The bridge wire is generally a resistance element that is usually formed of a very thin wire of circular cross section or a thin foil element, though other forms are possible, such as disclosed in U.S. Pat. No. 3,974,424, where the resistive element is of a generally flat S-shaped configuration having two arcuate resistor portions which are joined by a connecting portion. The bridge wire or resistance element generally has an electrical resistance which is appreciably greater than the electrical resistance of the lead wires. Therefore, passage of an electrical current through the leads and the resistive bridge element causes the later to be heated or to spark, and thereby firing the primary deflagration charge.
A well known safety hazard of such electro-explosive devices is that they can be accidently fired by static electricity. A safety requirement therefore is the “one ampere-one watt no fire” requirement. This requirement states that a device must be capable of dissipating one watt of power while one ampere of current is passed through the bridge element without firing. This indirectly fixes the desired combined resistance of the lead wires and the bridge element at one ohm. The combined lead wire and bridge element resistance is generally held between 0.9 and 1.1 ohms. If the total resistance is too low, the one watt requirement will not be met; if it is too high, excessive heating due to higher power dissipation will result.
The primary explosive is sensitive in that it will explode with relatively little stimulus—in a deflagration effect, i.e. creating a flame front by its explosive decomposition—a relative low energetic explosion in which flame front moves relatively slowly though the energetic material in contrast to a detonation type explosive. The purpose of this relatively low energetic explosion is to subsequently ignite a more energetic transition or secondary explosive material—that is more powerful than the primary explosive; but, which creates a detonation, i.e. an explosive effect marked by the propagation of a shock wave that transverses the explosive material. The shock wave from a secondary explosive can typically reach thousands of meters per second and create the desired significant effect. The stimulus (such as the bridge wire), the primary explosive and the secondary explosive form an “explosive train.” Often, there is an intermediate sensitive, intermediate explosive force, transition explosive between the primary and secondary explosive materials—to ensure that the explosive train functions smoothly.
Current modern weapons systems designs require bridge wire EED's which are rugged, to withstand the acceleration and/or spin forces applied thereto when incorporated in high military speed shells or rockets. Which bridge wire EED's must further meet diverse requirements—such as multi-point initiation, non-traditional configurations (including substrates which are not flat), and different voltage availability. Finally, and importantly, the bridge wire EED's must be miniaturizable, to conform to modern systems miniaturization for size and mass reduction—utilizing micro-electromechanical fuze systems (MEMS) based technology and processes, which are based upon lithographic techniques, or offshoot techniques such as plating, molding, plastic injection, and/or ceramic casting—examples of such applications of MEMS technology are disclosed in a series of patents to Robinson and Robinson et al., including U.S. Pat. No. 6,167,809, issued Jan. 2, 2001; U.S. Pat. No. 6,568,329, issued May 27, 2003; U.S. Pat. No. 6,964,231, issued Nov. 15, 2005; and, U.S. Pat. No. 7,316,186, issued Jan. 8, 2008. Typical processes for loading energetic materials (e.g. pressing, melt-pouring, or curing) are incompatible with the desire to reduce component size and mass. Further, such applications often involving handling of dry energetic material, which presents safety concerns.
There is a need in the art for a method of forming rugged bridge wire EED's that can meet the various safety requirements for an electro-explosive device and be easily miniaturized and tailored to conform to multi-point initiation, non-traditional configurations (including curved, i.e. non-flat, substrates) and different voltage availability—bridge wire EED's that have smaller footprints and that can be integrated into a MEMS dimensioned devices—and that are easy to manufacture.